Process for ultrafiltration of stabilized emulsions

ABSTRACT

PCT No. PCT/FR93/00577 Sec. 371 Date Apr. 18, 1994 Sec. 102(e) Date Apr. 18, 1994 PCT Filed Jun. 15, 1993 PCT Pub. No. WO93/25298 PCT Pub. Date Dec. 23, 1993.A process is provided for the ultrafiltration of stabilized emulsions, such as cutting oils, which comprises circulating the emulsion through a chamber partially bounded by a porous membrane, and before the emulsion enters the said chamber, a small quantity of salt is introduced into it, corresponding to a weight ratio of salt/oil present in the emulsion between 0.01 and 0.2.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for ultrafiltration ofstabilized emulsions, for example of used cutting oils, and moreparticularly to such a process consisting in introducing a smallquantity of salt into the emulsion with a view to destabilizing it, thatis to say without preliminary separation of the water and of the oilbefore its delivery to the ultrafiltration membrane.

2. Description of the Related Art

A cutting oil is a mixture of mineral oil, of surface-active agents, ofcosurfactants and of many various additives (bactericides, extremepressure agents, lubricants, corrosion inhibitors, wetting agents andthe like). This combination of constituents which are emulsifiable inall proportions with water is commonly employed at concentrations whichvary from 1 to 10% of oil per 90 to 99% of water. Cutting oil emulsionsare employed in all operations for machining and shaping metals andcutting stones in order to provide the following functions at thecutting tool:

lubrication and reduction in friction,

cooling,

reduction in wear and corrosion,

removal of the impurities (swarf, dust etc.).

These emulsions operate in a closed circuit on machine tools until theylose their effectiveness over some months because of a slow bacterialdegradation and because of being contaminated with impurities. They mustconsequently be replaced at regular intervals. The organic pollutioncaused by a direct discharge of these spent emulsions into naturalsurroundings can be damaging to the environment. The problem of thetreatment of these spent cutting oil emulsions must therefore be faced.

Stabilized emulsions cannot be treated by conventional methods ofseparation which are employed for unstabilized emulsions, namely: phaseseparation, flotation or coalescence with a particle or fibre bed orhydrocyclones, because the oil droplets are too small to be capable ofbeing separated by gravity separation. In addition, the presence of thesurface-active agents and of the cosurfactants prevents any coalescenceof the oil droplets because of the existence of an electrical and/ormechanical barrier.

The treatment methods which are employed at present can be classifiedinto three categories:

Treatment processes using a thermal route: Two types of treatment can bedistinguished; the simplest one consists of direct burning of the spentemulsion; the other type is based on an evaporation. The aqueous phaseis thus evaporated off and the oil is recovered at the end of theoperation. These two methods are adapted to all types of cutting fluidbut have a major disadvantage, namely a very high energy consumption.

Physicochemical treatment processes: These processes are based on adestabilization of the spent oil-water emulsion, which is often referredto as "breaking the emulsion". This breaking is generally obtained bythe action of chemical reactants of acidic, salt or polyelectrolytetype; the subsequent separation of the water and of the oil is generallycarried out by simple phase separation. An example of this type ofbreaking process is described in document FR-A-2,656,812. Althoughresulting in good separation efficiencies after quite long phaseseparation periods, these processes exhibit two major disadvantages. Thefirst disadvantage is related to the large quantities of reactants to beemployed and is reflected in the substitution of an acidic or salinepollution for an initial organic pollution. The second majordisadvantage of these techniques is that the breaking is comparable to achemical reaction and is therefore found to be stoichiometric, that isto say that it requires an optimum dosage of the reactants employed. Itis therefore essential to carry out tests as during a flocculation inorder to determine beforehand the optimum concentrations of salts, acidsor polyelectrolytes to be employed.

Ultrafiltration processes: In order to separate the oil from the waterby ultrafiltration, the emulsion containing cutting oil droplets with adiameter of less than 5 μm is circulated through an ultrafilter equippedwith a water-permeable porous membrane whose pores have a diameter ofapproximately 100 Å. Treatment processes employing ultrafiltrationexhibit undoubted advantages. They consume only little energy, thetreatment plants are small in size and, after treatment, the water isfree from cutting oil. In addition, no human maintenance is neededcontinuously, as in the case of the physicochemical processes andconsequently they can be easily automated and this is a considerableadvantage in the present context. However, this ultrafiltrationtechnique involves limitations which are related especially to theviscosity of the emulsion which will have a direct effect on the flow ofthe permeate and the formation, in the course of time, of a so-called"polarization" layer on the membrane which is produced by the gradualaccumulation of the oil droplets. When this layer contains from 30 to40% of oil, it has the consistency of a particularly viscous whitishgel. The blocking thus obtained causes a very appreciable decrease inthe water-permeability of the membrane, that is to say a reduction inthe flow of permeate passing through it and the elimination of thecapillary separation action in the blocked regions of the membrane,which is reflected in a leakage of oil and poor oil-water separation. Toovercome this major disadvantage of the ultrafiltration method it isknown to lower the viscosity of the emulsion either by diluting it or bycompletely or partially breaking the emulsion by introducing salts,organic compounds or acids or simply by diluting the emulsion. In theprior art this chemical destabilization is accompanied by a phaseseparation preceding or following the ultrafiltration. This type ofdestabilization prior to the ultrafiltration consists in treating twodifferent phases, the oil separated off beforehand and the residualemulsion to be ultrafiltered, the viscosity of which has thus beenreduced.

These improvements in the prior art for the separation of stableemulsions by ultrafiltration nevertheless do not make it possible todevelop these ultrafiltration processes economically on an industrialscale because two stages are needed to improve the flow of permeatethrough the membrane, one of these being destabilization by breaking theemulsion. Furthermore, an oil pollution is replaced with a considerablesaline pollution in the case of which no favourable solution isavailable. In addition, human supervision is necessary and a process ofthis type can be only partially automated.

The present invention is therefore aimed at improving theultrafiltration processes using a membrane in order to improve theirperformance, that is to say to increase the flow of permeate, to limitthe problems of blocking and of formation of the polarization layer, andto reduce the membrane areas used at present, but to do this in a singlestage, without preliminary destabilization by breaking the emulsion intotwo phases, of "oil" and "lower viscosity emulsion".

SUMMARY OF THE INVENTION

The subject of the present invention is therefore a process forultrafiltration of stabilized "oil-in-water" emulsions, consisting incirculating the emulsion through a chamber partially bounded by a porousmembrane, characterized in that, before the emulsion enters the saidchamber, a small quantity of salt is introduced into it, correspondingto a weight ratio of salt/oil present in the emulsion of between 0.01and 0.2.

Halogens and formates of alkaline-earth metals may be found among thesalts which are suitable for the process according to the invention. Ina preferred embodiment of the invention, calcium and magnesium halidesand formates will be preferred.

Nevertheless, the preferred, the most common place and the least costlysalt is calcium chloride.

To make use of the process according to the invention, the concentrationof calcium chloride in the emulsion would be chosen to be between 0.1and 1 kg/m³.

The advantage of the process according to the invention lies in the factthat, in contrast in what a person skilled in the art does, only a smallquantity of salt is introduced, at most ten times smaller than thatneeded to produce a destabilization with breaking of the emulsion intooil and dilute emulsion. In the said process of the invention there isno breaking of the emulsion before or in the separation chamber.Consequently, no film of oil is deposited on the surface of the membraneand therefore no polarization gel is formed. The membrane is thereforetransformed into a surface coalescer which permits the continuousseparation of the coalesced oil at the surface of the membrane. Thequantity of oil which is separated corresponds stoichiometrically to thequantity of oil present in the ultrafiltrate recovered and which iscoalesced on the said membrane. As a result, the oil concentration inthe circuit for concentrating the ultrafiltrate remains constant andlow, in contrast to those described in the state of the art and this,bearing in mind the low viscosity of the medium, favours the productionof high permeate flow rates.

An explanation of the phenomenon used by the invention would consist insaying that the presence of a low salt concentration simultaneouslypromotes the separation of the microdroplets and of the water holdingthe emulsion and their coalescence at the membrane under the combinedeffect of the flow pressure and of the hydrodynamic forces, the saltremaining trapped by the membrane. No pollution, be it saline or oily,is possible in this process, and this offers big advantages whencompared with the techniques developed by those skilled in the art.Thus, the water recovered is of low hardness and is comparable with somesurface waters meeting the standards of drinkability which is required.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of nonlimiting example withreference to the attached drawings, in which:

FIG. 1 is a diagrammatic view of an ultrafiltration unit permitting theuse of the process according to the invention;

FIG. 2 is a diagrammatic view of a laboratory cell permittingultrafiltration tests to be made;

FIG. 3 is a graph of the change in the flow rate of ultrafiltrate as afunction of the time, produced with the cell of FIG. 2;

FIG. 4 is a graph of the ratio Q/Qo as a function of the CaCl₂concentration;

FIG. 5 is a graph of the change in the flow rate as a function ofpressure; and

FIG. 6 is a graph of the change in the flow rate of ultrafiltrate as afunction of time, produced with the unit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an ultrafiltration unit 10 comprising a tank 12 intended tocontain the emulsion 14 to be treated and provided with a coolingcircuit 16. The tank 12 comprises an exit 18 which is connected, bymeans of a centrifugal pump 20, to a filtration circuit 22. A secondarycircuit 24, provided with a valve 26, is arranged between the pump 20and the tank 12 and allows the flow of fluid in the filtration circuit22 to be controlled.

The filtration circuit 22 comprises two valves 28 and 30 which arearranged upstream and downstream of an ultrafiltration cell providedwith a membrane 33, which includes a filtrate exit 34. The filtrationcircuit 22 additionally comprises a separator 36 downstream of theultrafiltration cell 32. When the ultrafiltration unit is used the fluidto be treated travels at a high speed through the cell 32, parallel tothe membrane 33.

In the first embodiment of the invention, in order to destabilize theemulsion, the ultrafiltration process consists in adding smallquantities of a salt 31 to the emulsion before it enters the cell 32.The salt is preferably CaCl₂.

To study the influence of the addition of CaCl₂ on the flow rate ofultrafiltrate, tests were carried out at different salt concentrationsin the case of an emulsion containing 4% of oil. The salt concentrationis expressed in mg of salt added to one liter of emulsion.

These tests were carried out with the laboratory cell shown in FIG. 2.This cell, which is of the "Amicon" type, is shown generally at 40 andincludes a stirrer 42. Air under pressure is conveyed from a storagevessel 44 into the cell 40 by means of a conduit 46 which is providedwith a valve 48. As in the example of the ultrafiltration unit in FIG.1, the cell 40 is provided with a membrane 50. The ultrafiltrate flowsthrough a conduit 52 towards a balance 54.

FIG. 3 shows the change in the flow rate of ultrafiltrate as a functionof time at different concentrations of CaCl₂. The results shown in FIG.3 were produced with the cell of FIG. 2 and show a clear improvement inthe flow rate of ultrafiltrate at salt concentrations which are higherthan 300 mg/l. At the end of the operation, free oil floats at thesurface of the retentate.

Although the polarization layer is not removed completely, its effectsare attenuated as the salt concentration increases. The calcium chloridehas the effect of lowering the potential between the oil droplets; as aresult, as the water passes through the membrane, the oil droplets whichare concentrated at the latter coalesce and separate from the emulsion.This free oil rises to the surface, and this destabilizes thepolarization layer and prevents any gel formation.

The change in the ratio of the flow rate of ultrafiltrate to the flowrate of pure water at 1 bar and 20° C., Q/Qo, is plotted in FIG. 4 as afunction of the calcium chloride concentration at different oilconcentrations. These tests were also carried out with the cell of FIG.2. Three plateaus can be discerned in each curve; on the first plateauthe flow rate remains equal to that obtained without salt addition. Anabrupt increase in the flow rate is then observed until the secondplateau; the emulsion is then partially destabilized but no oil-waterseparation is observed; the optimum salt concentration corresponds tothe establishment of this plateau. On the third plateau, the flow rateof ultrafiltrate is equal to that of pure water; the salt concentrationcorresponding to this plateau is equal to the quantity necessary for acomplete breaking given by the literature.

                  TABLE A                                                         ______________________________________                                        [Oil] (%)    4           6       8                                            Opt. [CaCl.sub.2 ] (g/l)                                                                   0.5         0.75    1                                            [CaCl.sub.2 ]/[Oil]                                                                        0.125       0.125   0.125                                        Q(i)         159         130     125                                          ______________________________________                                    

The table summarizes the results obtained. The quantity of salt which isnecessary increases with the oil concentration in the emulsion butremains well below the quantities used to destabilize the treatedemulsions by a physicochemical route. In all cases the flow rate ofpermeate is relatively high when compared with that obtained withoutdestabilization. The ratio of the optimum quantity of salt to the oilconcentration is constant; this makes it possible to determine for anyemulsion to be treated the quantity of salt which is necessary foroperating under optimum ultrafiltration conditions.

The influence of pressure on the flow rate of ultrafiltrate in the caseof an emulsion containing 4% of oil and a salt concentration of 400 mg/lis shown in FIG. 5 and was investigated with the cell of FIG. 2. Up to1.4 bars the permeate flow increases with pressure; a decrease in theflow is then observed at high pressures. Above 3 bars the flow becomesequal to the flow obtained using conventional ultrafiltration.

FIG. 6 shows the change in the flow rate of ultrafiltrate as a functionof time in the case of an oil emulsion at a concentration of 4%. Thesetests were carried out with the ultrafiltration unit of FIG. 1. 5 g ofCaCl₂ were added to 10 liters of emulsion after 4 minutes'ultrafiltration. As can be seen in the graph, the flow rate ofultrafiltrate increases instantaneously from 115 to 210 l/hm² and thenremains practically constant.

With the process according to this first embodiment the residualpollution is minimal because the quantities of salt which are added arevery small.

According to a second embodiment of the invention it will beadvantageously possible to treat the emulsion before it enters theseparation chamber without visible addition of salt, solely by dilutingwith tap water containing a low calcium concentration. In thisembodiment, the emulsion is diluted to an oil concentration of 1%. Sucha diluted emulsion becomes unstable and can be separated using thefiltration unit described above.

The process according to this second embodiment offers the additionaladvantage, besides that of not adding any salt, of being applicable toany type of stabilized emulsion.

We claim:
 1. A process for ultrafiltration of stabilized oil-in-wateremulsions, comprising circulating the emulsion through a chamberseparated by a porous membrane, wherein before the emulsion enters saidchamber, alkaline earth metal halide salt is introduced into saidchamber in an amount corresponding to a weight ratio of salt/oil presentin the emulsion of between 0.01 to 0.2.
 2. The process according toclaim 1, wherein the salt is an alkaline earth metal chloride.
 3. Theprocess according to claim 2, wherein the salt is calcium chloride. 4.The process according to claim 3, wherein the amount of calcium chlorideintroduced into the emulsion produces a concentration in said emulsionbetween 0.1 to 1 kg/m3.
 5. Process according to claim 2, wherein thesalt is magnesium chloride.